Vehicle tire including tread portion having profile

ABSTRACT

A vehicle tire which has an aspect ratio of less than about 55% and comprises a tread portion having a profile whose radius of curvature decreases substantially continuously from the tire equator to each tread edges. The tread profile from the tire equator to each tread edge is substantially defined by a curve such as ellipse, cycloid, epicycloid, involute curves and the like, of which equation is differentiable in the range between the tire equator and each tread edge.

The present invention relates to a vehicle tire more particularly to animproved tread portion suitable for low aspect ratio tires being capableof improving the wear resistance and steering stability.

Recently, low aspect ratio tires are widely used in passenger carsespecially high performance cars.

In general, if the radius of curvature of the tread is increased, thesteering stability during high speed straight running may be improved.But, as the tread camber decreases, and also a camber angle is given bywheel alignment, the ground pressure is increased in the shoulderportion during cornering. Accordingly, uneven wear is liable to occur inthe shoulder portion. Further, the high speed durability decreases whenthe tire is mounted on the wheel with a relatively large camber angle.

Therefore, in order to solve these problems, a relatively large camberis given to the shoulder portion by forming the shoulder portion with anarc having a small radius of curvature of about 200 to 300 mm whereasthe tread crown portion is formed by an arc having a larger radius ofcurvature of not less than 1000 mm. That is to say, the tread profile isformed by a double radius curve.

In such a profile, even if the two arcs, crown arc and shoulder arc areconnected with each other without forming any inflection point, theground pressure increases in the vicinity of the connecting pointbetween the arcs, and as a result uneven wear is liable to be caused,and sometimes the steering stability during high speed straight runningand cornering performance are deteriorated.

It is therefore, an object the present invention to provide a vehicletire in which the ground pressure distribution is evened to improve thewear resistance and running performance.

According to one aspect of the present invention, a vehicle tirecomprises a tread portion of which tread radius of curvature decreasessubstantially continuously from the tire equator to the tread edges.

Here, the tread edges are defined as the axially outermost edges of theground contacting patch of the tire which is mounted on a standard rimand inflated to a standard pressure and loaded with a standard load. Thestandard rim is the "Standard Rim" specified in JATMA, "Measuring Rim"in ETRTO, "Design Rim" in TRA or the like. The standard pressure is themaximum air pressure in JATMA, "Inflation Pressure" in ETRTO, maximumpressure given in the "Tire Load Limits at Various Cold InflationPressures" table in TRA or the like. The standard load is 88% cf themaximum load, which is the maximum load capacity in JATMA, "LoadCapacity" in ETRTO, maximum value given in the above-mentioned table inTRA or the like.

Preferably, the tread profile is defined by a curve which is a part ofthe locus of an equation such as elliptic equation, cycloid equation,epicycloid equation, involute equation and the like, which equation isdifferentiatable in the range of variables corresponding to the rangefrom the tire equator to each tread edge.

Embodiments of the present invention will now be described in detail inconjunction with the accompanying drawings.

FIG. 1 is a cross sectional view of a tire according to the presentinvention showing an example of the internal tire structure.

FIG. 2 is a diagram for explaining an elliptic equation.

FIG. 3 is the locus of an elliptic equation for the tread curve.

FIG. 4 is a diagram for explaining an epicycloid equation.

FIG. 5 is the locus of an epicycloid equation for the tread curve.

FIG. 6 is a diagram for explaining a cycloid equation.

FIG. 7 is the locus of a cycloid equation for the tread curve.

FIG. 8 is a diagram for explaining a method of forming a tread curve bymultiple arcs to approximate the loci.

FIG. 9 shows footprints of test tires according to the invention.

FIG. 10 shows footprints of a test tire according to a prior art.

FIG. 11 is a graph showing the ground pressure distributions of the testtire.

FIG. 12 is a diagram for explaining an involute curve of which the basecircle is an ellipse.

FIG. 13 shows an example of the tread profile based on an involutecurve.

FIG. 14 shows a tread profile which approximates the involute curve ofFIG. 13.

FIG.15 shows the tread profile of a test tire according to a prior art.

FIG. 16 shows the tread profile of reference tire 1.

FIG. 17 shows the tread profile of reference tire 2.

FIGS. 18 and 19 show footprints of test tires.

In the drawings, vehicle tires according to the present invention are apneumatic radial tire for passenger cars. The aspect ratio (sectionheight H/section width W) is not more than 55% in this example 45%.

As shown in FIG. 1, the tire comprises a tread portion 5, a pair ofsidewall portions 4, a pair of bead portion 3 with a bead core 2therein, a carcass 6 extending between the bead portions 3 through thetread portion 5 and sidewall portions 4, and a belt 7 disposed radiallyoutside the carcass 6 in the tread portion 5.

The carcass 6 is composed of at least one ply in this example a singleply of cords arranged radially at an angle of 70 to 90 degrees withrespect to the tire equator C. For the carcass cords, organic fibercords, for example nylon, polyester, rayon, aromatic polyamide and thelike are used.

The belt 7 comprises at least one ply of parallel steel cords laid at anangle of 10 to 35 degrees with respect to the tire equator C. In thisexample two cross plies 7A and 7B called steel breaker are disposed.Aside from steel cords, high elastic modulus organic fiber cords made ofrayon, aromatic polyamide or the like can be used. Further, it ispossible to dispose a band ply 9 radially outside the breaker 7A, 7B soas to cover at least axially outer edges of the breaker to prevent theirlifting during high speed running. The band ply 9 is preferably made ofrelatively low modulus organic fiber cords, such as nylon, and the cordangle is 0 to 5 degrees with respect to the tire equator C.

The tread portion 5 is usually provides with tread grooves for waterdrainage although such grooves are not shown in FIG. 1.

In the meridian section of the tire under the standard condition, thetread profile FL is defined by a curve L whose radius decreasessubstantially continuously from the tire equator PC to each tread edgeE. Usually, the tread profile FL is symmetrical about the tireequatorial plane C. But it is possible to design the profileasymmetrical if necessary. Here, the standard condition is that the tireis mounted on the standard rim J and inflated to the standard pressurebut loaded with no tire load.

The above-mentioned tread curve L is defined by an elliptic function, acycloid function preferably epicycloid function, an involute function orthe like. In such a function, accordingly, the continuously decreasingradius means that the equation is differentiatable in the range of thevariables (x and y) corresponding to the actual range of the treadprofile between the tire equator PC at the tread surface and each treadedge E.

Taking a typical size of 235/45R17 for low aspect ratio pneumatic tiresfor passenger cars, a way of applying such a function to the actualtread profile will be explained more concretely.

First, the tread camber Y is determined to satisfy that the arctangent##EQU1## of the tread camber YE at the tread edge E and a half treadwidth (TW/2) becomes equal to the actual camber angle in use which isusually about 3 to about 5 degrees for example about 4 degrees; and thatthe arctangent ##EQU2## of the tread camber YC at the TW/4 point P1 anda quarter tread width (TW/4) is in the range of from about 1 to about 2degrees for example about 2 degrees.

The tread width TW is the axial distance between the above-explainedtread edges E.

In the above-mentioned tire size, for example, the camber YE at thetread edge E is 7.6 mm, the camber YC at the mid point P1 is 1.3 mm, anda half tread width TW/2 is 100 mm,

Elliptic Curve

FIG. 2 shows an ellipse, the basic elliptic equation of which is##EQU3## a>b.

In order to apply the elliptic equation to the tread curve L, the x-yaxes are shifted as follows. The x-axis is an axial line extending fromthe tire equator PC towards the tread edge E. The y-axis is a radialline extending radially inwardly from the tire equator PC at the surfaceof the tread portion. The origin is thus at the tire equator PC on thesurface of the tread portion.

In this coordinate system, the tread curve L must pass on two points ofwhich x and y values are (x=50, y=1.3) and (x=100, y=7.6) because thecamber values YC and YE are determined as above.

Hence, the equation of the tread curve L may be written as ##EQU4##

The locus of this equation 2 which is described when the x value variesfrom 0 to 100 is used as each half of the tread curve L extending fromthe tire equator PC to each tread edge E. Thus tread profile issymmetrical about the tire equator. The tread camber Y (mm) at an axialposition X (mm) is as follows, and the locus is shown in FIG. 3.

    __________________________________________________________________________    X 5 10                                                                              15                                                                              20                                                                              25                                                                              30                                                                              35                                                                              40                                                                              45                                                                              50                                                                              55                                                                              60                                                                              65                                                                              70                                                                              75                                                                              80                                                                              85                                                                              90                                                                              95                                                                              100                                   __________________________________________________________________________    Y 0 0 0.1                                                                             0.2                                                                             0.3                                                                             0.5                                                                             0.6                                                                             0.8                                                                             1 1.3                                                                             1.6                                                                             1.9                                                                             2.3                                                                             2.8                                                                             3.3                                                                             3.8                                                                             4.5                                                                             5.3                                                                             6.3                                                                             7.6                                   __________________________________________________________________________

Epicycloid Curve

FIG. 4 shows an epicycloid. The epicycloid is the locus of a point N seton the circumference of a circle (d) rolling on the circumference of afixed base circle (c) of which center is placed on the origin O of x-ycoordinates. The basic epicycloid equation is

    x=(r+R)cos θ+R cos θ'

    y=(r+R)sin θ+R sin θ'                          (3)

wherein

θ is the angle between the x-axis and a straight line drawn between thecenter of the rolling circle and the center of the fixed base circle,and

θ is the angle between the x-axis and a straight line drawn between thepoint N and the center of the rolling circle. In FIG. 4, the angles θand θ' are counted counterclockwise from the x-axis.

Since it is a premise that there is no slip between the rolling circleand base circle, we may write ##EQU5##

From this equation,

    k=r/R                                                      (5)

    θ'=(k+1)θ-π/2k                              (6)

By the equations 6 and 3, ##EQU6##

To apply this equation to the actual tread curve L, the x-y axes areshifted in the same way as the above-mentioned elliptic function.

By the equation 7, the tread camber Y may be write as

    Y=A×(r+2R-y)                                         (8)

(A=constant)

As explained above, the tread curve L must pass on two points of which xand y values are (x=50, y=1.3) and (x=100, y=7.6).

Hence, from the equations 7 and 8 and these conditions, the tread curveL may be written in the shifted x-y coordinate system as ##EQU7##

In this example, k=0.4, but it is possible to set at different value.

The locus of this equation (x=0 to 100) is used as each half of thetread curve L from the tire equator PC to each tread edge E. Thus thetread profile is symmetrical about the tire equatorial plane. The treadcamber Y (mm) at an axial position X (mm) is as follows and the locus isshown in FIG. 5.

    __________________________________________________________________________    X  0.1                                                                             9 21.5                                                                             33.7                                                                             45.4                                                                             56.4                                                                             66.7                                                                             75.9                                                                             84                                                                              90.9                                                                             96.5                                                                             100                                          __________________________________________________________________________    Y  0 0 0.2                                                                              0.6                                                                              1.1                                                                              1.7                                                                              2.5                                                                              3.4                                                                              4.3                                                                             5.4                                                                              6.6                                                                              7.6                                          __________________________________________________________________________

Cycloid Curve

FIG. 6 shows a cycloid. The cycloid is the locus of a point N set on thecircumference of a circle (h) rolling on a straight line (i). The basiccycloid equation is

    x=π·r-r(θ-sin θ)

    y=r(1+cos θ)                                         (10)

wherein

r is the radius of the rolling circle (h),

θ is the angle between the y-axis and a straight line drawn between thepoint N and the center of the rolling circle. The x-axis is an axialline extending from the tire equator PC towards each tread edge E. They-axis is a radial line extending radially inwardly from the tireequator PC. The origin is thus the tire equator PC. There is no need toshift the x-y axes as in the former two cases.

By the equation 10, the tread camber Y may be written as

    Y=A×r(1+cos θ)                                 (11)

(A=constant)

From the equations 10 and 11 and the conditions that the tread curve Lmust pass on two points (x=50, y=1.3) and (x=100, y=7.6), the treadcurve L may be written as

    x=31.85×π-31.85(θ-sin θ)

    y=0.019×31.85(1+cos θ)                         (12)

The locus of this equation is shown in FIG. 7. The tread camber Y (mm)at an an axial position X (mm) is as follows.

    __________________________________________________________________________    X 0.1                                                                             9 21.5                                                                             33.7                                                                             45.1                                                                             55.7                                                                             65.3                                                                             73.7                                                                             80.9                                                                             86.9                                                                             91.5                                                                             95                                                                              97.4                                                                             98.9                                                                             99.7                                                                             100                               __________________________________________________________________________    Y 0 0 0.2                                                                              0.5                                                                              1  1.6                                                                              2.2                                                                              2.9                                                                              3.7                                                                              4.4                                                                              5.2                                                                              5.8                                                                             6.4                                                                              6.9                                                                              7.3                                                                              7.6                               __________________________________________________________________________

According to the present invention, the tread curve L must be decreasedcontinuously from the equator to the tread edges. However, it is notalways necessary to strictly follow the locus of the above-explainedequations. Slight aberrations may be permitted. In other words, as shownin FIG. 8, a curve composed of multiple arcs so as to approximate thelocus may be used as far as the aberrations are small for example withinplus minus 1/10 mm. This will be advantageous to making the tirevulcanizing mold. If importance is attached to such easiness of makingthe mold, the number of the arcs is in the range of from about 7 asshown in FIG. 8 to about 18.

Comparison Test

The following test tires were made and the ground pressure distributionwas measured under the camber angle of 0 degree and 4 degrees.

Test tires: Ex.1 of which the tread profile is based on the epicycloidequation 9; Ex.2 based on the elliptic equation 2; and Conventional tireof which the tread profile is defined by a triple radius arc composed ofa crown arc Rc (1100 mm), middle arc Rm (400 mm) and shoulder arc Rs(100 mm).

Tire size: 235/45R17

Wheel rim size: 17X8JJ (standard rim)

Inner pressure: 230 KPa

Tire load: 400 kgf

Tread camber: (X=50 mm Y=1.3 mm) and (X=100 mm Y=7.6 mm)

The footprints obtained by the tests are shown in FIGS. 9 and 10,wherein the area of which the ground pressure was not less than 5.0kgf/sq.cm is shaded.

FIG. 11 shows the area in percentage to the total ground contacting areaas a function of ground pressure.

As apparent from FIGS. 9 and 10, in the tires according to theinvention, the area of which the ground pressure is not less than 5.0kgf/sq.cm is small in comparison with the conventional tire. Further, asshown in FIG. 11 in the tires according to the invention, the groundpressure of the widest area was 4.75 kgf/sq.cm, which is remarkablylowered when compared with the conventional tire which showed 5.25kgf/sq.cm. Thus the occurrence of uneven wear can be effectivelyprevented.

Further, Example tire 1 was installed on a Japanese FR car with a 2500cc turbocharged engine, and tested for the straight running stability,lane change stability, cornering performance, and high speed durabilityaccording to ECE30 (camber angle=0 and 4 degrees). The test results areas follows.

    ______________________________________                                        Tire                Ex. 1  Conv.                                              ______________________________________                                        Straight running stability                                                                        8      6                                                  Lane change stability                                                                             7      6                                                  Cornering performance                                                                             8      6                                                  High speed durability                                                         Camber angle = 0 deg.                                                                             110    100                                                Camber angle = 4 deg.                                                                             98     100                                                ______________________________________                                    

The first three performances were evaluated into ten ranks wherein thestandard is six. The high speed durability is indicated by an indexbased on that the conventional tire is 100. The larger the value, thebetter the performance. From the tests, it was confirmed that Exampletire 1 was remarkably improved in the straight running stability andcornering performance when compared with the conventional tire.

Further, using Japanese 2500 cc FR cars provided with Example tire 1 andConventional tire, the average wear was measured after running for 9000km to obtain the running distance per 1 mm wear. Further, the shoulderwear (wear in shoulder portion/wear in crown portion) was also measured.The test results are as follows.

    ______________________________________                                        Tire                   Ex. 1  conv.                                           ______________________________________                                        Running distance (km) per 1 mm wear                                                                  103    100                                             Shoulder wear          105    100                                             ______________________________________                                    

In this table, an index based on that the conventional tire is 100 isused. The larger the value, the better the performance.

Involute Curve

FIG. 12 shows still another embodiment of the present invention, whereinthe tread profile is such that the center of the radius R of the treadcurve L describes an ellipse D. That is, the tread curve L is aninvolute curve of which the base circle is an ellipse D. The involutecurve is the locus which is described by one end of a fixed lengththread 10 of which the other end is fixed to the origin O when thethread 10 is wound around the ellipse (or unwound) without slack. Inthis example, in the same way as the former examples, the radius Rdecreases continuously from the tire equator PC at the surface of thetread portion to the tread edge E. In this case, it is possible or easyto determine the tread profile by fist giving radii of curvature to twopositions such as the tire equator and tread edge.

To obtain an equation of the ellipse D, x-y coordinates are set asfollows. The origin 0 is on the equatorial plane C at a predetermineddistance from the surface of the tread portion at the tire equator PCwhich distance corresponds to the radius Rc of the tread curve L at thetire equator PC. The x-axis extends axially outwardly from the tireequator C in parallel with the tire axis. The y-axis extends radiallyoutwardly along the equatorial plane C. In this coordinate system, theelliptic equation is ##EQU8## wherein y≧0

x=0˜a

a=constant

b=constant

a≠b.

In this case, the radii of curvature at two points are determined in thefirst place. For example, when the tire size is 235/45R17 and the treadwidth TW=206 mm, the radius Rc at the tire equator PC is set as 1250 mm,the radius at the tread edge E is set as 52 mm. From the equation 13,the ellipse D as the base circle of involute curve is ##EQU9##

By this equation, the tread camber YE at the tread edge is given as 8.6mm. The tread curve L described is shown in FIG. 13.

FIG. 14 show another example of the tread curve L which is composed ofmultiple arcs to approximate the tread curve shown in FIG. 13. In thisexample, a half tread curve is composed of nine arcs and the whole curveis composed of seventeen arcs. In this case, as explained above, themaking of the vulcanizing mold becomes easier.

Aside from the above-explained accurate involute curve, such a pseudoinvolute curve that the length of the thread 10 is for example longerthan the original length measured from the origin O to the point Pe(x=a, y=b) along the circumference of one quarter of the ellipse D.Further, it is also possible to set slightly shorter. In other words, itis not always necessary that the length of the thread 10 is equal to thecurve length between the origin and the point Pe.

Comparison Test

The following test tires were made and the ground pressure distributionwas measured.

Test tires: Exs.11 and 12 of which the tread profile is defined by aninvolute curve; Ex.13 of which the tread profile is defined by a curvecomposed of nine arcs shown in FIG. 14; Conventional tire of which thetread profile is defined by a curve composed of three arcs shown in FIG.15; Ref.11 of which the tread profile is defined by a curve composed ofeight arcs shown in FIG. 16: Ref.12 wherein as shown in FIG. 17 acentral tread part extending 30 mm from the tire equator is defined by aconstant radius of 1100 mm, and the axially outer part thereof isdefined by a variable radius R which is

    R=(7×10.sup.-4 ×e.sup.(2.5×10.spsp.-2.sup.×X) +7.3×10.sup.-5 ×X).sup.-1

Tire size: 235/45R17

Tread width: TW=206 mm

Wheel rim: 17×8JJ (standard rim)

Inner pressure: 230 KPa

Tire load: 400 kgf

Camber angle: 0 and 3 degrees

The footprints of the test tires are shown in FIGS. 18 and 19, whereinthe area of which the ground pressure was not less than 5.0 kgf/sq.cm isblacked out. The narrower the blacked area, the better the groundpressure distribution.

Further, in the same way as explained above, the tires were tested forthe straight running stability, lane change stability, corneringperformance, wear resistance, high speed durability, and rollingresistance. The results are as follows.

    ______________________________________                                                                    Ex.  Ref. Ref.                                    Tire        Ex. 11  Ex. 12  13   11   12   conv.                              ______________________________________                                        Rc (mm)     1250    1127.5  1250 --   --   --                                 a (mm)      115.2   115.2   *    --   --   --                                 b (mm)      1231.9  1111.17 *    --   --   --                                 Stability                                                                     Straight running                                                                          7       5       7    5    6    5                                  Lane change 6       6       6    5    5    5                                  Cornering   6       6       5    5    5    5                                  Wear resistance                                                               Running distance/                                                                         106     103     105  100  98   100                                1 mm wear                                                                     Shoulder wear                                                                             105     110     103  100  95   100                                ECE30 High speed                                                              durability                                                                    camber angle = 0 deg.                                                                     107     110     107  103  103  100                                camber angle = 4 deg.                                                                     103     108     102  99   96   100                                Rolling resistance                                                                        96      94      97   100  97   100                                ______________________________________                                         *) approximate Ex. 11 tread curve                                        

From the tests, it was confirmed that Example tire 11 was greatlyimproved in the straight running stability, cornering performance andwear resistance in comparison with conventional tire. Furthermore, therolling resistance was decreased.

What is claimed is:
 1. A vehicle tire comprising a tread portion havinga profile whose radius of curvature decreases substantially continuouslyfrom the tire equator at the surface of the tread portion to tread edgesand whereinthe tread profile in the range from the tire equator at thesurface of the tread portion to each said tread edge is defined by acurve which is substantially an elliptic curve, the tread edges areaxially outermost edges of the ground contacting patch of the tire whenthe tire is mounted on a standard rim and inflated to a standardpressure and loaded with a standard load, and in the following x-ycoordinates: the origin is set on the surface of the tread portion atthe tire equator; the x-axis extends from the tire equator towards eachtread edge in parallel with the axis of the tire; and the y-axis extendsradially inwards along the tire equator from the surface of the treadportion, the equation of the elliptic curve is ##EQU10##
 2. A vehicletire comprising a tread portion having a profile whose radius ofcurvature decreases substantially continuously from the tire equator atthe surface of the tread portion to tread edges and, wherein the treadprofile in the range from the tire equator at the surface of the treadportion to each said tread edge is defined by a curve which issubstantially an involute curve of which the base circle is anellipse,the tread edges are axially outermost edges of the groundcontacting patch of the tire when the tire is mounted on a standard rimand inflated to a standard pressure and loaded with a standard load, andin the following x-y coordinates: the origin is on the equatorial plane(C) of the tire and spaced from the surface of the tread portion by adistance which equals a radius Rc of the tread curve at the tire equator(PC); the x-axis extends in parallel with the tire axis toward each sideof the equatorial plane; and the y-axis extends radially outwardly alongthe equatorial plane, the elliptic equation of the base circle is##EQU11##
 3. The vehicle tire according to claim 2, wherein saidconstants (a) and (b) are such values that the length measured from apoint where x=a and y=b to the origin along the elliptic curve is equalto the radius Rc.